Examples of elements used for power semiconductor devices such as inverters include Schottky barrier diodes (SBDs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and insulated gate bipolar transistors (IGBTs). Power semiconductor devices each containing silicon (Si) as a semiconductor material, have been gradually approaching their limits of the theoretical physical properties of silicon in recent years. Hence, attention is now drawn to compound semiconductor materials, such as silicon carbide (SiC) and gallium nitride (GaN) that go beyond the limit of the theoretical physical properties of silicon. Accordingly, power semiconductor devices containing such semiconductor materials are under development.
Some compound semiconductors have lattice constants smaller than the lattice constant (5.43 Å) of Si (1 Å=0.1 nm). Examples of such compound semiconductors include GaN (a=3.189 Å, c=5.185 Å), 3C—SiC (4.36 Å), 4H—SiC (a=3.07 Å, c=10.05 Å), and 6H—SiC (a=3.08 Å, c=15.1 Å). A semiconductor substrate having a small lattice constant has a small coefficient of diffusion of ions as doped; in addition, the semiconductor substrate needs to undergo heat treatment for activating the doped ions (annealing for activation) at a high temperature of 1500° C. or higher. Hence, the annealing for activation needs to be performed prior to forming a pattern of an insulating film (especially, Si-based insulating film) on the semiconductor substrate. Accordingly, the pattern formation of the insulating film desirably has a small effect on an impurity diffusion region, which is previously formed.
For example, Patent Document 1 discloses patterning of a field insulating film that is formed on a SiC substrate and has a thickness ranging from about 100 to 600 nm, where the patterning includes the following processes: first, the field insulating film undergoes dry etching until it becomes several ten nanometers in thickness; and the field insulating film then undergoes wet etching.